Packed beds of rock using air as the heat transfer fluid are potentially suitable for thermal energy storage in concentrating solar power plants. However, the pressure drop through packed beds is strongly dependent upon particle shape, roughness and packing arrangement. This is particularly true for crushed rock, which is asymmetric, rough and randomly packed. In order to design rock beds and determine if they are less costly than existing thermal storage systems, it is necessary to estimate the pressure drop and associated pumping power. Empirical friction factor correlations obtained from a number of sets of crushed rock are presented for two different packing directions. For practical reasons, they are expressed in terms of the particle volume-equivalent sphere diameter, which is easy to measure. A simplified Nusselt number correlation for air-rock beds, also expressed in terms of the volume-equivalent sphere diameter, is given for heat transfer calculations. These correlations are intended to allow for a quick and straightforward estimate of rock bed pressure drop and heat transfer characteristics, without the necessity of measuring particle shape, roughness or size distribution; they are; consequently, limited in their applicability to materials similar to those used for the original tests. (C) 2015 Elsevier Ltd. All rights reserved.